All measurements were taken with an output voltage of 1 volt. I performed the same measurements
with an output voltage of 2 volts and achieved similar results
with only slightly higher THD+N measurements. All measurements shown
are with the stock volume pot installed. Installation of the Shallco
did not make any noticeable changes except where noted below.

When looking at the following measurements, it is interesting to note the
accented second-ordered harmonic. It is even more interesting to note the
absence of higher order harmonics. This is one of the characteristics that make this tube
design so
pleasing to listen to. Even-order harmonics tend to sound very natural
and add a warmth to the sound, where as odd-order harmonics have a tendency
to sound harsh and unnatural.

The first two graphs show THD+N figures of 0.03% for both a 1 kHz and 10 kHz
test tone. This is very respectable for a tube-based preamp..

IMD came in at 0.0007% with the standard volume pot installed. With
the Shallco installed, IMD dropped to levels below our test equipment
capabilities (0.0000%).

The frequency response shows no tendency to roll-off at either end of the
audible frequency spectrum and is quite flat from 10 Hz to beyond 70 kHz..

Conclusions

As you can tell, I enjoyed using the BC21.1. The NOS (New Old Stock, which
means the tubes are old but unused) tubes allowed me to
personalize the sonic characteristics to match my tastes. I personally found
the look and feel of the BC21.1 to be very pleasing, and I could live with
this preamp in a two-channel system for years to come. I think a number of
high-end two-channel manufacturers should spend some R&D budget on building
usability into their products rather than adding another oversized cap or
shorter signal paths. I am glad to see Blue Circle has taken the initiative
of providing a minimal remote option. However, I would love to see a preamp
design similar to the BC21.1 with a remote control that has discrete input
selection, power, volume, and mute controls. A few high end two-channel
manufactures have already started down
this path, and I encourage others to follow.